FIG. 1 shows a prior art OFDM receiver 100. Digital signals are generated by media access controller MAC 102 and associated circuitry, which generate a stream of digital payload data information for forming the payload a wireless packet. A preamble is pre-pended by function 104 and a packet header including data rate, encoding type, packet length and other information is added by header insertion function 108, and the content is scrambled 112, encoded 116, punctured 118, interleaved 120, and modulated into a plurality of subcarriers 122. An IFFT function 124 converts the complex valued frequency domain modulation information provided by 122 into a complex valued time domain waveform having I and Q components, which is filtered 126, converted to an analog signal 127, and low pass filtered 132 to remove out of band components introduced by the conversion, modulated to a carrier frequency such as 2.4 Ghz by mixer 128 and transmit oscillator 130, and applied to a power amplifier 134, which is coupled to an antenna 136 for transmission through the wireless media.
FIG. 2 shows the 64 OFDM complex subcarriers 200, each of which is generated by an output of the OFDM modulator 122 and provided as an input to the IFFT 124. The OFDM modulator generates 48 data subcarriers which carry the modulated signal, and these complex data subcarriers are accompanied by a DC subcarrier, 4 pilot subcarriers, and 6 null subcarriers below and 5 null subcarriers above the data subcarriers. The pilot subcarriers generate continuous signals for frequency tracking, and the null subcarriers represent subcarrier channels that are unused. The use of these subcarriers is described in the 802.11 series of IEEE wireless Local Area Network (LAN) standards, including specifically 802.11a and 802.11g, all of which are incorporated by reference.
FIG. 3 shows the simple case of a single subcarrier 302 which is represented as having a real and imaginary component, which generate the complex sine waveform 400 with real and imaginary components as shown in FIG. 4, where the waveform generated isA(t)=A*exp(2*pi*k*Δf*t)                where A is the complex constellation point,        Δf is the subcarrier spacing,        and k is the subcarrier index.        
FIG. 5 shows the frequency domain representation of an OFDM symbol 500, and FIG. 6 shows the time domain representation 600 of the amplitude of the same symbol. A problem arises in an OFDM transmission system whereby the OFDM symbol stream may produce time-domain signals with an unusually high Peak to Average Power Ratio (PAPR), as shown in the peaks 602 and 604 of FIG. 6. The gain of the power amplifier 134 of FIG. 1 is linear over a signal range as shown in the transfer function 706 of FIG. 7. An input signal 704 generates an amplified output signal 702 until a critical input amplitude 708 is reached, where the curve 706 flattens out because of saturation and other non-linearities in the amplifier. When the input signal exceeds threshold 708, the output response is no longer linear, and inter-modulation cross products are generated by the power amplifier which distort the time and frequency domain representations of the desired symbol, and these distortion generate errors at a receiver which attempts to demodulate the nonlinear subcarriers. In order to avoid the saturation and non-linear operation of the power amplifier 134, the overall signal amplitude is scaled by a factor which allows for peaks 602 and 604 to remain below the linear operation threshold 708 of the power amplifier.
U.S. Pat. Nos. 6,512,797, 6,512,797, 6,853,632 and 6,928,084 describe PAPR reduction systems whereby the information subcarriers are modified to reduce PAPR.
U.S. Pat. No. 6,741,661 describes a PAPR reduction system for CDMA2000 cellular telephone applications whereby the information subcarriers are modified using a raised cosine function.
U.S. Pat. No. 6,950,389 describes a PAPR reduction system using differential phase encoding of the signal to be transmitted.
U.S. Pat. No. 6,985,533 describes a PAPR reduction system using transforms at the transmitter and receiver which cause the transmitted signal to have a reduced PAPR.